Simplify prekey id generation

ts/util/wrappingAdd.std.ts

@@ -0,0 +1,12 @@
+// Copyright 2026 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// Add two unsigned 24-bit integers and return truncated 24-bit result
+export function wrappingAdd24(a: number, b: number): number {
+  if (!Number.isFinite(a) || !Number.isFinite(b)) {
+    throw new Error('Invalid arguments');
+  }
+
+  // eslint-disable-next-line no-bitwise
+  return (a + b) & 0xffffff;
+}